Electric machine controller

Abstract

A controller for a switched electric machine, especially a switched reluctance motor, takes timing information from a rotor position transducer to generate a switch-off output at a point near maximum phase inductance in a phase inductance cycle. A switch-on signal is generated after a delay but still within the phase inductance cycle. A simple form of single-pulse control is thereby achieved. A comparator is also provided which monitors phase winding current. A pulse generator is actuated by the comparator when the winding current exceeds a reference level and is used to control the motor in a chopping mode at low speeds and is disabled by the comparator at higher speeds when the single-pulse control is used.

What is claimed is: 
     
       1. A controller for an electric machine, comprising a stator defining stator poles and having at least one phase winding, a rotor defining rotor poles, and switch means for controlling current in the winding, the controller comprising: a transducer for deriving rotor position information;   a timer operable to produce switch-on and switch-off signals to which the switch means are responsive, the timer being arranged to receive a first output from the transducer indicative of a position of one of the rotor poles relative to one of the stator poles coincident with a switch-off point in the phase inductance cycle for operating the machine in one of a motoring and a generating mode, and to produce the switch-off signal in response thereto; and   first delay means responsive to a second output from the transducer within the phase inductance cycle to generate a switch-on signal, subsequent to the switch-off signal, after a delay.   
     
     
       2. A controller as claimed in claim 1 in which the first and second outputs from the transducer are the same output. 
     
     
       3. A controller as claimed in claim 1 in which the second output is a point in the phase inductance cycle towards minimum phase inductance when the machine is motoring and maximum phase inductance when the machine is generating. 
     
     
       4. A controller as claimed in claim 3, including a comparator having a first input for a signal indicative of a winding current and a second input for a signal indicative of a current reference value, the comparator being operable to generate a trigger signal in response to the winding current being in excess of the current reference value, a pulse generator responsive to the trigger signal to generate a further switch-off signal of predetermined duration and a further switch-on signal to which the switch means are also responsive, and gate means by which the switch-on signal for the switch means is at least dependent upon the switch-on signals from both the pulse generator and the first delay means. 
     
     
       5. A controller as claimed in claim 3, including a comparator having a first input for a signal indicative of a winding current and a second input for a signal indicative of a current reference value, the comparator being operable to generate a trigger signal in response to the winding current being in excess of the current reference value, a pulse generator responsive to the trigger signal to generate a further switch-off signal of predetermined duration and a further switch-on signal to which the switch means are also responsive, and gate means by which the switch-on signal for the switch means is at least dependent upon on the switch-on signals from both the pulse generator and the first delay means, the controller further including second delay means arranged further to delay the second output from the transducer to the gate means. 
     
     
       6. A controller as claimed in claim 5 in which the trigger signal output of the comparator is connected directly with the gate means, such that the switch-on signal is inhibited when the first input is indicative of the winding current exceeding the current reference value. 
     
     
       7. A controller as claimed in claim 4 in which the pulse generator is arranged to generate the switch-off signal as an adjustable pulse of predetermined minimum duration. 
     
     
       8. A controller as claimed in claim 7 in which the transducer is arranged to produce a pulse output, having a leading edge and a trailing edge. 
     
     
       9. A controller as claimed in claim 8 in which the first and second outputs from the transducer are the same output and the output from the transducer is one of the leading and trailing edges of the said pulse-output, the switch-on signal being independent of the other of the leading and trailing edges of the pulse. 
     
     
       10. A method of single pulse control of a switched reluctance machine comprising a stator, having at least one phase winding, a rotor and switch means associated with the phase winding, the method comprising: timing actuation of the switch means to switch off current to the phase winding to a point coincident with a switch-off point in the phase inductance cycle for operating the machine in one of a motoring and a generating mode, and thereafter timing actuation of the switch means to switch on current to the phase winding after a delay.   
     
     
       11. A method as claimed in claim 10, including initiating the start of the delay at a later point in the phase inductance cycle. 
     
     
       12. A method as claimed in claim 11, wherein the switch means comprises first and second switches, and wherein the method further includes the steps of timing switching off the first switch to a first timed delay; and then further delaying switching off the second switch and allowing the phase current to freewheel when the first switch is open and the second switch is closed. 
     
     
       13. A method as claimed in claim 12, further including chop controlling the machine by comparing a maximum reference current value with a phase winding current; generating a switch-off signal when the phase winding current exceeds the reference current value; and gating the switch-off of the current to the phase winding in accordance with the phase inductance period. 
     
     
       14. A method as claimed in claim 13 in which the output of the machine is varied by adjusting the duration of the delay above a predetermined minimum period. 
     
     
       15. A method as claimed in claim 14 including deriving timing pulses from rotation of the rotor. 
     
     
       16. A method as claimed in claim 14 including deriving timing pulses from the rotation of the rotor in which the timing of the actuation of the switch means to switch-off current to the phase winding is dependent on the occurrence of one of the leading and trailing edges of the said timing pulse, and timing actuation of the switch means to switch-on current to the phase winding is independent of the other of the leading and trailing edges of the pulse. 
     
     
       17. A switched reluctance drive including a switched reluctance machine comprising a stator, having at least one phase winding, a rotor, a transducer for generating rotor position information, switch means for switching current to the winding and a controller, the controller comprising: a comparator operable to generate a trigger signal in response to a first input indicative of a winding current exceeding a current reference value;   a logic gate arranged to transmit switch-off and switch-on signals to switch means;   and a pulse generator responsive to the trigger signal to generate the switch-off signal of predetermined duration to which the switch means are responsive;   a timer circuit operable to generate further switch-on and switch-off signals to which the switch means are also responsive, the timer circuit being arranged to receive a first output from the transducer indicative of a position of one of the stator poles relative to one of the stator poles coincident with a switch-off point in the phase inductance cycle for operating the machine in one of a motoring and a generating mode, and to generate the switch-off signal in response thereto; and   delay means responsive to a second output from the transducer within the phase inductance cycle to generate the switch-on signal, subsequent to the switch-off signal, the switch means actuation signal from the logic gate being dependent upon switch-on signals being received together from the pulse generator and the delay means.